Fix null PacketEvents player in AFK packet limiter

[HyacinthHaru]

修复了 #24

修复 LimitedPacketEfficiency 中 PacketEvents 发送事件无玩家时的空指针异常（NPE）

服务器日志中反复出现来自 LimitedPacketEfficiency$PacketListeners.onPacketSend 的异常，根本原因是对 e.getPlayer() 缺少 null 检查。

摘要

• 修复了 LimitedPacketEfficiency 在处理数据包发送时，玩家为空的边界情况。
• 在某些发送路径中，PacketSendEvent.getPlayer() 可能返回 null，之前直接传入 limiting.contains(...)，导致触发空指针异常。
• 通过在玩家为空时提前返回，并在后续 AFK 检测和距离计算中复用已验证的玩家实例来解决问题。
• 在避免 PacketEvents 监听器反复抛出异常的同时，保持对正常玩家相关数据包的 AFK 限流行为不变。

[Copilot]

Pull request overview

This PR fixes a recurring NPE in the AFK packet limiter by handling the edge case where PacketEvents fires a PacketSendEvent without an associated Bukkit Player.

Changes:

• Add a null-check for e.getPlayer<Player>() and early-return when absent.
• Reuse the validated player instance throughout the packet-type handling logic.
• Refactor helper methods to take Player explicitly (instead of re-fetching from the event).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.